If history is any guide, the Ferguson, Mo., police officer who shot an unarmed black teenager on Aug. 9 is not likely to be charged -- much less convicted -- for the shooting.

Officer Darren Wilson left town in the days since he shot Michael Brown while multiple federal and local investigations are ongoing. The Brown family's attorneys have argued that the findings of their independent autopsy showed that Wilson should have already been arrested.

But he hasn't been -- and those familiar with the history of police-involved deaths say that a convergence of both U.S. law and cultural norms put the odds against it.

"It is really hard to convict a police officer. They get a super presumption of innocence," Laurie Levenson, a former federal prosecutor who is now a law professor at Loyola University in Los Angeles, told TPM. She was involved in a grand jury investigation of an officer-involved death, she said, but it never went to trial.

"We don't want to believe that the people we hire to protect us could be the people who want to harm us," she said, "and so we give them a huge benefit of the doubt."

Hard numbers are hard to come by, but Levenson and others agreed that it is exceedingly rare for a police officer to be indicted for a homicide committed in the line of duty. Convictions are even rarer. The FBI reported 410 justifiable homicides by law enforcement in 2012. The number of indictments appear to be minimal after a TPM review of available press reports. A 1979 study found three convictions out of the 1,500 police killings it studied over a five-year period.

There are many reasons why -- including the fact that police officers are frequently justified in shooting an assailant. But the law also provides police officers with an additional layer of protection. The U.S. Supreme Court ruled in Graham v. Connor in 1989 that actions by law enforcement must be judged by a standard known as "objective reasonableness."

Per Justia, that means their actions "must be judged from the perspective of a reasonable officer on the scene, and its calculus must embody an allowance for the fact that police officers are often forced to make split-second decisions about the amount of force necessary in a particular situation."

The net effect is that there is a higher threshold for prosecuting police officers for actions committed in the line of duty compared to your run-of-the-mill homicide.

"It's just the law. When we dance around it, we dance around our own involvement. The law is made in our name," Eugene O'Donnell, a former police officer and prosecutor, who is now a lecturer at the John Jay College of Criminal Justice, told TPM. "When they're wearing the mantle of authority that they're given, they have a lot of authority."

People protest on Sunday, Aug. 17, for Michael Brown, who was killed by a police officer last Saturday in Ferguson, Mo. (AP/Charlie Riedel).

Another factor that is impossible to ignore in the context of the Michael Brown shooting is race. As Mother Jones reported last week, research has routinely found that people of color are disproportionately involved in incidents of violent action by police officers.

In addition, Levenson and David Klinger, a former police officer who is now a professor at the University of Missouri-St. Louis, agreed that jurors are already largely predisposed to believe police officers. Whether it is comfortable to admit or not, that inherent trust in the police is frequently tied to race. White jurors are more likely to sympathize with a white police officer, Levenson said, and that in itself complicates the prosecution of officer-involved shootings. She pointed to the infamous acquittal of Los Angeles police officers for the beating of Rodney King -- a majority-white jury and white police officers -- as just one high-profile example.

"Reasonableness is through the eyes of your life experiences, and of course, that includes things like race. We don't like to talk about it a lot because our country is really bad about talking about race," Levenson said. "A black member of the community might see police actions differently than a white member of the community because they have a different experience with the police."

When you combine those factors with others, like the often close working relationships between police and prosecutors, it all helps to explain the notable dearth of police officers being charged and convicted for homicides committed in the line of duty.

And so while the St. Louis County prosecutor's office has said that a grand jury could begin reviewing evidence and testimony in the Brown shooting as soon as this week, that is also why history says that an indictment for Wilson is unlikely to be the result.

"You have to remember that a prosecutor is not going to bring an indictment, and shouldn't bring an indictment, unless he or she has a reasonable prosecutorial belief that they can prove beyond a reasonable doubt that the police officer has committed a crime," said Klinger, who added that he has testified as an expert on behalf of officers who have been indicted on use-of-force charges.

And given the difficulty in making that case with officer-involved shootings, Klinger concluded: "The vast, vast majority of police shootings do not result in an indictment."